1. Field of the Invention
The present invention relates to a laser beam machining system by which laser beam machining with a uniform depth can be applied to a work.
2. Description of the Related Art
In the process of manufacturing a semiconductor device, a plurality of regions are demarcated by planned split lines called streets which are arranged in a lattice pattern on a face side of a roughly circular disk-like semiconductor wafer, and devices such as ICs and LSIs are formed respectively in the demarcated regions. Then, the semiconductor wafer is cut along the streets to split it into the regions provided with the devices, thereby producing individual semiconductor chips. In order to manufacture devices with smaller size and higher functions, a module structure has been put to practical use in which a plurality of semiconductor chips are stacked and bonding pads of the stacked semiconductor chips are connected to each other. The module structure is so configured that the semiconductor wafer is provided with through-holes (via holes) at its portions provided with electrodes, and the through-holes (via holes) are filled up with a conductive material, such as aluminum and copper, for connection with the electrodes (refer to, for example, Japanese Patent Laid-open No. 2003-163323).
The through-holes (via holes) provided in the semiconductor wafer as above-mentioned are generally formed by use of a drill. However, the through-holes (via holes) provided in the semiconductor wafer have small diameters of 100 to 300 μm, and boring by use of a drill has the problem of poor productivity. In order to solve this problem, the present applicant has proposed in Japanese Patent Application No. 2005-64867 a laser beam machining system by which minute holes can be efficiently formed in a work such as a semiconductor wafer.
The laser beam machining system thus proposed includes machining feed amount detection means for detecting the relative machining feed amounts of a chuck table holding a work thereon and laser beam irradiation means, storage means for storing the X and Y coordinate data on minute holes to be formed in the work, and control means for controlling the laser beam irradiation means on the basis of the X and Y coordinate data on the minute holes stored in the storage means and a detection signal sent from the machining feed amount detection means, wherein the work is irradiated with one pulse of a laser beam when the portion, corresponding to the X and Y coordinate data on the minute hole to be formed in the work, of the work has been brought to a position directly under a condenser of the laser beam irradiation means. However, in order to form a through-hole in the work, the same portion of the work must be irradiated with a pulsed laser beam a plurality of times. Therefore, the use of the above-mentioned laser beam machining system is not necessarily satisfactory in regard of productivity, since movement of the work must be carried out a plurality of times.
In order to meet the above-mentioned requirement, the present applicant has proposed in Japanese Patent Application No. 2005-362236 a laser beam machining system including laser beam irradiation means including acousto-optical deflection means using an acousto-optical device, wherein a laser beam oscillated by laser beam oscillation means is deflected when passing through the acousto-optical device, whereby the same work position of the work is irradiated with the laser beam while performing machining feeding of the work.
In the method of forming a laser beam-machined hole by irradiating a semiconductor wafer with a laser beam from the back side of the semiconductor wafer as above-mentioned, the same portion of the semiconductor wafer must be irradiated with the laser beam a plurality of times, and the irradiation must be so controlled as not to open a hole in the electrode, called bonding pad, formed on the face side of the semiconductor wafer. However, the energy distribution of the laser beam is a Gaussian distribution such that the energy is strongest at the center and is decreased toward the outer peripheral area. Therefore, the machining proceeds most at the central portion of the laser beam with which the work is irradiated, so that it is impossible to form a laser beam-machined hole with a uniform depth. Thus, there is the problem that the electrode (bonding pad) is melted at the central portion of the laser beam, resulting in formation of a hole there.